This invention relates to a coaxial cable type of sensor for detecting the leakage of a liquid and, more particularly, to a liquid leakage sensor capable of detecting the leakage of a liquid more quickly than sensors of the prior art and capable of reducing variations in the time required for the detection thereof.
Heretofore, oils such as crude oil, petroleum, gasoline, kerosene, organic solvents, chemicals and the like have been transported from one place to another through underground piping. Various methods have been proposed for detecting leakage in pipes in these liquid transportation systems. One of the methods proposed, for example, comprises installing along a pipeline a pair of conductors separated from each other by an insulating material which is porous to the liquid being transported and allows passage of the liquid therethrough causing the electrical characteristics between the conductors to change when the liquid leaks out of the pipe and enables one to detect the place where the liquid is leaking.
In the aforementioned method, a liquid leakage sensor comprising a pair of coaxial conductors has been used as shown in FIG. 1. Therein, an internal conductor 1 and an external braided metallic conductor 3 which is porous to the liquid to be detected is shown, the conductors being arranged coaxially, and a porous insulator 2 of continuously porous polytetrafluoroethylene (PTFE) and the like is provided as a detector layer between the conductors. A protective polyester braid 5 is provided and acts as a protective layer on the periphery of the external braided conductor 3. In the liquid leakage sensor thus constructed, a leak of a liquid such as petroleum passes through the external braid 5 and porous PTFE insulator 4, through the openings in the braid of conductor 3, and permeates through the internal porous PTFE insulator 2 used as a detector layer, causing the relative permittivity of the detector layer 2 to change. Such a detector was the subject of a prior patent application of the present inventors, which was published as Japanese Patent Application Laid-Open Gazette No. 54-15435 and registered as Patent No. 984263. Such a liquid leakage sensor was capable of detecting liquid leakage by measuring the change in permittivity or by means of a pulse reflection distortion based on the change in the permittivity. The liquid leakage sensor thus contructed offered capabilities allowing for the position and range of leakage to be detected, the types of oils to be distinguished to a certain degree, and oil and water to be selectively differentiated. However, the prior art detector still has drawbacks in that 20 to 100 minutes can be required to detect oil leakage and there are variations in detection time in the longitudinal direction of the cable.
In view of the aforementioned problems, it is an object of the present invention to provide a liquid leakage sensor capable of detecting leakage quickly and minimizing variations in detection time in the longitudinal direction.